1. Field of the Invention
The present invention relates to an image coding method for high efficiency coding of motion image signals for transmission or recording of such motion image signals, and to an apparatus for performing the image coding method.
2. Description of the Prior Art
In the image coding apparatus, recently, together with the development of the television telephone and the television conference system, various high efficiency coding technologies have been realized. In particular, coding technology making use of inter-frame prediction coding is widely used in the image coding apparatus.
The inter-frame prediction coding system determines a prediction signal by predicting a frame to be encoded from another frame, and encodes a prediction error which is a difference between the frame to be encoded and the prediction signal. The prediction signal is determined using the following methods. (1) A reproduction signal of a frame before the intended frame to be encoded is delivered from a memory of a local decoder, and is compensated for motion to obtain a prediction signal. (2) A reproduction signal of a frame before the intended frame to be encoded and a reproduction signal of a frame after the intended frame are delivered from memories of a local decoder, and after they are compensated for motion, a mean value of them is determined as a prediction signal.
In the method (2), in other words, a frame located between two reproduction signals apart from each other by two frames or more is determined by motion compensating frame interpolation, and this frame is used as the prediction signal. One such prior art image coding apparatus is disclosed in EP90304841. The conventional image coding apparatus is explained below.
The motion picture signals entering the image coding apparatus are subjected to two types of prediction methods alternately on a frame by frame basis. The prediction method is the manner of determining the prediction signal. The first prediction method includes reading the reproduction signal two frames before from a frame memory of a local decoder, and compensating the read signal for motion to obtain a prediction signal, and the second predict method includes determining an image signal by motion compensating frame interpolation from preceding and succeeding reproduction frames as a predicting signal. Afterwards, a prediction error signal is determined which is a difference between the frame to be encoded and the prediction signal, and an encoded prediction error signal is the output signal of the image coding apparatus. The image coding apparatus, possessing a local decoder, decodes the code, determines the reproduction image, and stores it in a frame memory.
In such a configuration, however, a reproduction frame later in time is needed when determining the prediction signal from the reproduction frames before and after the frame to be coded, that is, the frame later in time must have been already coded. To realize this, it is necessary to change the time sequence of the frames of the motion picture signals entering the coding apparatus, and therefore the configuration of the coding apparatus is complicated.